Under the current paradigm, immune function in vertebrates is characterized as consisting of two major types of responses, the adaptive immune response and the innate immune response. The adaptive immune response is present only in vertebrates and provides selective long-term protection from re-infection by pathogens to which there has been a previous exposure. The innate immune response provides an immediate response to infection or injury and promotes local phagocytic, cytolytic, and cytotoxic actions of the effector cells.
The innate immune system is evolutionarily the oldest (>250 million years old) immune system in animals and is present in virtually all vertebrates and invertebrates. The innate arm of the vertebrate immune response is the first line of defense against invading pathogens and is composed of immune cells that are largely found in the tissue compartment, particularly in the tissues of the major portals of entry for bacterial and viral pathogens such as the skin and the respiratory and digestive tracts. Mediated by a set of germline-encoded receptors, the immune cells of the innate arm of the immune response are phenotypically and genotypically distinct from the immune cells that mediate the actions of the adaptive arm of the immune response.
The innate immune response and inflammation are intimately related. The inflammatory response to localized infection or trauma is generally regarded as beneficial resulting in pathogen containment and clearance by the cytotoxic and phagocytic actions of the tissue resident effector cells. Whereas local inflammation is beneficial, systemic inflammation is a deleterious and often fatal consequence of traumatic injury or septicemia. Thus, limiting the inflammatory response to trauma or septicemia is the subject matter of current medical research, and restricting the innate immune response during traumatic injury or septicemia may be an effective way of limiting inflammation.
The Tissue Factor or extrinsic clotting cascade has been recently identified as an activator of clotting during systemic inflammation or septicemia. Coagulation has traditionally been viewed as a separate process from inflammation that evolved specifically to prevent the loss of blood volume and blood constituents as a result of a physical breach of the circulatory system. In this traditional view, thrombin mediates the cascade that is the sole mechanism for blood coagulation. However, recent evidence has established that pro-inflammatory cytokines, particularly IL-6, also promote coagulation through increased expression of Tissue Factor (TF) which is now thought to be the principal activator of clotting during systemic inflammation or septicemia.
Conversely, Thrombin has a variety of pro-inflammatory actions to include leukocyte adhesion, pro-inflammatory cytokine production (particularly IL-6 and IL-8), soluble CD40L release from platelets, and histamine release from mast cells. Platelets, too, are previously unrecognized as inflammatory mediators with molecular signaling capabilities that link inflammation and the innate immune response. Thus, it is now clear that coagulation, inflammation, and the innate immune response are inter-related processes. The role of inflammation in the host immune response is poorly understood, yet it is an important aspect of regulation, control, and the interrelation of the innate arm of immune response. In a unified view, the mammalian innate immune response is a continuum of four stages beginning with inflammation, up regulation of the innate immune effector cells, down regulation of the cytotoxic and cytolytic actions of the innate immune response, and a return to homeostasis. A better understanding of the inflammatory cascade is central to an understanding of the immunopathology that underlies many of the most significant human diseases as well as the rational design of safe and effective therapeutic strategies.
T cells are a central player in the mammalian immune response and have been classified by the differential expression of T cell receptors (TCR) as either αβ or γδ heterodimers. The majority of circulating T cells in humans are of the αβ subset (αβT cells). Research has historically been focused on the αβ T cells, and their role in the adaptive immune response has been clearly established by their recognition of antigens via antigen presenting cells and TCR binding with the major histocompatability complex (MHC).
The γδ subset of T cells are a less well studied cell lineage of tissue resident cells encompassing up to 50% of the tissue resident T cell population. In circulation however, γδ T cells are much less prevalent than their αβ cousins and constitute only about 1-5% of the circulating T cell population. The impact of γδ T cell cytokine release on macrophage and natural killer cell (NK cell) activation has recently been reported in a murine model. Briefly, mice deficient in γδ T cell have been shown to be less able to withstand bacterial infection and are characterized by disruption of macrophage homeostasis, reduced INF-γ production from NK cells, and increased bacterial growth. Antigen recognition by γδ T cells is MHC independent and these cells have a diverse TCR repertoire. Therefore, γδ T cell can be activated during a wide variety of infections. Upon activation, these cells can undergo clonal expansion to constitute up to 97% of the total T cell population using a mechanism for clonal expansion that appears to be similar to an adaptive-type immune response in vitro.
The transcriptional profile of γδ T cells in naive mice and mice infected with Yesernia pseudotuberculosis has been characterized indicating that murine γδ T cells constitutively expressed high levels of transcripts for Granzymes A, B, and RANTES and other cytotoxic mediators such as lymphotoxin b, Fas ligand, NKR-PLA, NKR-PLC, LAG-3 and 2B4 as well as the corresponding inhibitory receptors. However to the best of our knowledge, similar studies have not been undertaken with primate γδ T cells.
Innate immune cell recognition of pathogens is generally thought to be mediated by a set of germline encoded receptors referred to as pattern-recognition receptors (PRRs) on tissue resident monocytes. The most well studied PRR are the Toll-like receptors (TLR). Toll receptors are transmembrane receptors first identified in Drosophila, but a homologous family of TLRs was subsequently identified in humans. The intracellular domain of the TLRs is structurally closely related to the IL-1 receptor and is referred to as the Toll/interleukin-1 (TIR) domain. Drosophila Toll and human TLR share homologous intracellular signaling components consisting of four major components: (1) the adaptor proteins MyD88, (2) TOLLIP (Toll-interacting protein), (3) the protein kinase IRAK (IL-IR-associated kinase), and (4) TRAF6 (TNF receptor-associated factor 6).
TLRs recognize conserved regions of pathogen derived molecules, commonly referred to as pathogen-associated molecular patterns or PAMPs, such as bacterial cell wall lipopolysaccharide (LPS), Staphylococcus enterotoxin B (SEB), tetanus toxin antigen (TTA), double stranded viral RNA fragments, bacterial DNA, and flagellin. TLR activation ultimately leads to activation of NFκB and downstream chemokine production in macrophage and dendritic cells including IL-8, IP-10, MIP-1α and β, and RANTES, as well as an influx of NK cells and T cells at the site of infection. Other than fibronectin fragments, human TLRs have not been shown to recognize endogenous ligands in response to pathogen infection in a manner that is analogous to the Drosophila Spätzle processing.
Heat shock proteins (HSPs) are a highly conserved family of stress-induced proteins that are produced by mammalian cells and microbial pathogen alike, and HSPs have been implicated in the immunopathology of rheumatoid arthritis and atherosclerosis. The immunogenicity of the HSPs appears to be derived from antigenic peptides chaperoned by the HSPs. HSP-peptide complexes potentiate the antigenicity of the chaperoned peptides by several orders of magnitude as compared to a non-HSP peptide binding antigen such as albumin. A common receptor for the HSP-peptide complex has been identified as CD91, an α2 macroglobulin receptor on monocytes. Thus, the HSPs are an important inflammatory regulator protein in the innate immune response to pathogen infection.
Defensins and cathelicidins are small cationic anti-bacterial peptides with immunoregulatory properties that have recently been discovered. Human defensins and cathelicidins are derived from cells including neutrophils, monocytes, certain lymphocyte populations, keratinocytes, and bronchial epithelial cells. Defensins are 3,5-4 kDa cysteine-rich, cationic peptides that have an intricate tertiary structure that resembles the structure of chemokines. Cathelicidins are linear peptides, as exemplified by LL-37, derived from the C-terminal sequence of human CAP-18.
Various lipoproteins derived from bacterial cell membranes have been shown to activate macrophages, fibroblasts, and lymphocytes to induce an inflammatory response and are broadly considered to be pro-inflammatory in nature. For instance a lipoprotein from Escherichia coli has been characterized and was shown to be an activator of monocytes. Recently, a family of lipopeptides has been characterized from Mycoplasma organisms. The parent 2 kDa lipopeptide has been characterized as a potent activator of macrophages and is known as the macrophage activating lipopeptide i.e., MALP-2. These monocyte activating lipopeptides have certain key structural features in common. The mycoplasmal lipopeptides contain peptides of varying lengths and sequences, but all have an N-terminal cysteine. The lipid portion is esterified as a 2,3-diacoyloxypropyl thioether of the N-terminal cysteine. Thus, the N-terminal nitrogen is free. Further, the S-enantiomer is biologically active whereas the R-enantiomer is not. The active lipopetide derived from E. coli, on the other hand, is a tri-lipid variant wherein the third lipid is attached as an N-terminal amide.
The biological role of the diacylglycerols has been well described in the literature. For instance it is well known that diacylglycerols participate in the transport of lipids as triglycerides and in association with soluble proteins such as the apolipoproteins, are transporters of cholesterol. Diacylglycerols are also known to have an intracellular signaling function. Intracellular, membrane bound phosphatidyl inositol-4,5-biphosphate is cleaved by the actions of the enzyme phospholipase C to release two intracellular messenger molecules, inositol triphosphate and membrane bound diacylglycerol (specifically 1-stearoyl-2-arachidonoyl glycerol). Diacylglycerol activates protein kinase C which activates transcription factor NFκB to up regulate the gene expression of various cytokines and chemokines.